Relating metal bioavailability to risk assessment for aquatic species: Daliao River watershed, China

Geochemical distribution and speciation of Tl and other trace metals in upper Beijiang River in South China: Approach of in-situ DGT monitoring

Mining activities frequently result in severe contamination of river water. This study aimed to better understand the spatial distribution characteristics of Tl and other metals (e.g., Al, Cd, Co, Mn, Ni, Zn, Pb, V, As, Mo, and Sb), and to assess their risks to human health. Surface water samples were collected from the upper Beijiang River (South China) via grab sampling and the diffusive gradients in thin-films (DGT) technique. The concentrations of Tl measured by grab sampling and δ-MnO₂-DGT ranged from 0.045 μg L^-1 to 0.231 μg L^-1 and from 0.056 μg L^-1 to 0.131 μg L^-1, respectively. Most of the metals monitored were below the threshold levels allowed by the drinking water standard in China, except for As, Sb, and Mn at specific sampling sites. The concentrations of other metals measured by grab sampling were higher than those measured using the DGT technique because of the differences in speciation during these measurements. The hazard quotient (5.43 × 10^-4-8.0 × 10^-1 for grab sampling and 2.23 × 10^-4-2.8 × 10^-1 for DGT technique) for the monitored trace metals demonstrated minimal health risk to human beings. The pollution status of these toxic metals in the study area was generally acceptable. As was found to be potentially the most harmful metal in the studied area, with hazard quotients at some sampling sites calculated by grab sampling of >1. It has previously been suggested that As is the most important non-carcinogenic contaminant. The combination of grab sampling and the DGT technique provides a comprehensive understanding of trace metals, especially Tl, in terms of potential bioavailability and ecological assessment.

Seasonal variations of cadmium (Cd) speciation and mobility in sediments from the Xizhi River basin, South China, based on passive sampling techniques and a thermodynamic chemical equilibrium model

Understanding the speciation and mobilization mechanisms of potentially toxic metals in sediments is critical to aquatic ecosystem health and contamination remediation in urban rivers. In this study, chemical sequential extraction, a thermodynamic chemical equilibrium model (Visual MINTEQ ver. 3.1), diffusive gradient in thin films (DGT), and high-resolution dialysis (HR-Peeper) techniques were integrated to identify seasonal variations in cadmium (Cd) mobility in sulfidized sediments. Acid-soluble Cd was the dominant geochemical fraction in sediments, followed by residual, oxidizable, and reducible Cd. The DGT-labile Cd concentration was associated with various geochemical processes and was independent of the total concentration and geochemical fractionation of Cd in sediments. Sulfate reduction facilitated the formation of insoluble CdS and induced low Cd concentrations in sediment porewater. Sulfide oxidation was principally responsible for lowered porewater pH and elevated Cd concentrations in summer. Strongly acidic conditions promoted release of sediment Cd but might reduce the binding efficiency of Chelex resin gel for dissolved Cd, leading to underestimation of the mobility of sediment Cd. Sediments generally functioned as a sink for Cd in winter and shifted to acting as a source in summer, releasing Cd into the overlying water mainly as Cd-S complexes with high potential to migrate downstream.

Speciation of Inorganic Compounds in Aquatic Systems Using Diffusive Gradients in Thin-Films: A Review

The speciation of trace metals in an aquatic system involves the determination of free ions, complexes (labile and non-labile), colloids, and the total dissolved concentration. In this paper, we review the integrated assessment of free ions and labile metal complexes using Diffusive Gradients in Thin-films (DGT), a dynamic speciation technique. The device consists of a diffusive hydrogel layer made of polyacrylamide, backed by a layer of resin (usually Chelex-100) for all trace metals except for Hg. The best results for Hg speciation are obtained with agarose as hydrogel and a thiol-based resin. The diffusive domain controls the diffusion flux of the metal ions and complexes to the resin, which strongly binds all free ions. By using DGT devices with different thicknesses of the diffusive or resin gels and exploiting expressions derived from kinetic models, one can determine the labile concentrations, mobilities, and labilities of different species of an element in an aquatic system. This procedure has been applied to the determination of the organic pool of trace metals in freshwaters or to the characterization of organic and inorganic complexes in sea waters. The concentrations that are obtained represent time-weighted averages (TWA) over the deployment period.

Trace Metal Contamination Impacts Predicted Functions More Than Structure of Marine Prokaryotic Biofilm Communities in an Anthropized Coastal Area

Trace metal (TM) contamination in marine coastal areas is a worldwide threat for aquatic communities. However, little is known about the influence of a multi-chemical contamination on both marine biofilm communities' structure and functioning. To determine how TM contamination potentially impacted microbial biofilms' structure and their functions, polycarbonate (PC) plates were immerged in both surface and bottom of the seawater column, at five sites, along strong TM contamination gradients, in Toulon Bay. The PC plates were incubated during 4 weeks to enable colonization by biofilm-forming microorganisms on artificial surfaces. Biofilms from the PC plates, as well as surrounding seawaters, were collected and analyzed by 16S rRNA amplicon gene sequencing to describe prokaryotic community diversity, structure and functions, and to determine the relationships between bacterioplankton and biofilm communities. Our results showed that prokaryotic biofilm structure was not significantly affected by the measured environmental variables, while the functional profiles of biofilms were significantly impacted by Cu, Mn, Zn, and salinity. Biofilms from the contaminated sites were dominated by tolerant taxa to contaminants and specialized hydrocarbon-degrading microorganisms. Functions related to major xenobiotics biodegradation and metabolism, such as methane metabolism, degradation of aromatic compounds, and benzoate degradation, as well as functions involved in quorum sensing signaling, extracellular polymeric substances (EPS) matrix, and biofilm formation were significantly over-represented in the contaminated site relative to the uncontaminated one. Taken together, our results suggest that biofilms may be able to survive to strong multi-chemical contamination because of the presence of tolerant taxa in biofilms, as well as the functional responses of biofilm communities. Moreover, biofilm communities exhibited significant variations of structure and functional profiles along the seawater column, potentially explained by the contribution of taxa from surrounding sediments. Finally, we found that both structure and functions were significantly distinct between the biofilm and bacterioplankton, highlighting major differences between the both lifestyles, and the divergence of their responses facing to a multi-chemical contamination.

Impacts of copper and lead exposure on prokaryotic communities from contaminated contrasted coastal seawaters: the influence of previous metal exposure

Our understanding of environmental factors controlling prokaryotic community is largely hampered by the large environmental variability across spatial scales (e.g. trace metal contamination, nutrient enrichment and physicochemical variations) and the broad diversity of bacterial pre-exposure to environmental factors. In this article, we investigated the specific influence of copper (Cu) and lead (Pb) on prokaryotic communities from the uncontaminated site, using mesocosm experiments. In addition, we studied how pre-exposure (i.e. life history) affects communities, with reference to previous metal exposure on the response of three prokaryotic communities to similar Cu exposition. This study showed a stronger influence of Cu contamination than Pb contamination on prokaryotic diversity and structure. We identified 12 and 34 bacterial families and genera, respectively, contributing to the significant differences observed in community structure between control and spiked conditions. Taken altogether, our results point toward a combination of direct negative responses to Cu contamination and indirect responses mediated by interaction with phytoplankton. These identified responses were largely conditioned by the previous exposure of community to contaminants.

Evaluation of diffusive gradients in thin films (DGT) technique for speciation of trace metals in estuarine waters - A multimethodological approach

Understanding the potential bioavailability of trace metals (TM) in marine systems is of prime importance to implement adapted regulations and efficiently protect our coastal and estuarine waters. In this study Diffusive Gradients in Thin films (DGT) technique with two different pore size was used to evaluate the potentially bioavailable fractions (DGT-labile) of Cd, Co, Cu, Ni, Pb and Zn at various depths of a highly stratified estuary (the Krka River estuary, Croatia) both in winter and summer. DGT-labile concentrations were compared to (1) total dissolved concentrations, (2) concentrations of labile species measured by anodic stripping voltammetry (ASV-labile) for Cu and (3) concentrations derived by chemical speciation modelling. High correlation between dissolved and DGT-labile concentrations was found for all metals, except for Zn where contamination problems prevented reliable conclusions. Percentages of DGT-labile fractions over total dissolved concentrations were (AVG ± SD): 92 ± 3%, 64 ± 2%, 23 ± 5%, 61 ± 3% and 57 ± 6% for Cd, Pb, Cu, Ni and Co, respectively. No significant difference was found between trace metal concentrations measured with an open pore and restricted pore devices, implying the predominance of kinetically labile metal complexes smaller than 1 nm. For Cu, ASV-labile and DGT labile concentrations were highly correlated (0.97) with ASV-labile concentration being around 35% lower than that of the DGT-labile. Modelling of chemical speciation reliably predicted dynamic (free, inorganic and part of organic complexes) concentration of Cd, whereas dynamic concentrations of Cu and Pb were underestimated by 32% and 65%, respectively. In view of the relative simplicity of DGT devices, they are well suited for the monitoring effort of coastal waters, informing on potentially bioavailable concentrations of TM and thereby, helping to achieve good environmental status of coastal waters, as stipulated within the EU Water Framework Directive.

Biokinetics and subcellular distribution of metals in Daphnia magna following Zn exposure: Implication for metal regulation

Although many studies have addressed the effects of various physicochemical and biological factors on metal bioaccumulation in aquatic organisms, the influences of metal exposure history have drawn much less attention. In the present study, we investigated the effects of different Zn exposure regimes (concentration: 1-200 μg/L, duration: 1-7 d) on the subsequent biokinetics, metallothionien-like protein (MTLP) induction and subcellular distribution of Cd and Zn in Daphnia magna. Zn body burden increased significantly with elevated exposure concentrations, but was kept within a narrow range regardless of the 200-fold variation of Zn concentrations. Significant induction (7-14 folds) of MTLP by Zn exposure was evident, but was decoupled from the Zn body accumulation. Under different regimes of Zn exposure, Zn was evenly distributed in insoluble (cellular debris, intracellular organelles and metal-rich granules) and soluble fractions (heat sensitive protein and MTLP). However, >60% of Cd was bound with MTLP regardless of the exposure concentration and duration. The biokinetic processes including uptake from the dissolved phase, food assimilation and excretion of Cd and Zn were quantified with radioactive tracers. The uptake rate constants of Cd and Zn from the dissolved phase either remained comparable or increased following Zn pre-exposure, and the efflux of Zn increased by nearly two-fold to counteract the increased uptake from water. The dietary assimilation of Zn also decreased significantly in response to Zn exposure. However, Cd assimilation efficiency was kept relatively constant in each Zn pre-exposure regime. Our results showed that Zn exposure had significant influences on the biokinetics and physiology of daphnids. Daphnids attempted to maintain the Zn body burden within a narrow range by modification of biokinetic processes instead of subcellular distribution.

Prediction of the bioavailability of potentially toxic elements in freshwaters. Comparison between speciation models and passive samplers

The aim of this work is to predict the bioavailability of the Potentially Toxic Elements (PTEs) Cd, Pb, Hg, Ni, Cu, Zn, As, Cr and Se in 6 sites within the Ebro River basin. In situ Diffusive gradient in thin-films (DGTs) and classical sampling have been used and compared. The potentially bioavailable fractions of each PTE was estimated by modelling their chemical speciation using three programs (WHAM 7.0, Visual MINTEQ 3.1 and Bio-met), following the suggestions published in recent European regulations. Results of the equilibrium-based models WHAM 7.0 and Visual MINTEQ 3.1 indicate that As, Cd, Ni, Se and Zn, predominate as free metals ions or forming inorganic soluble complexes. Copper, Pb and Hg bioavailability is conditioned by their affinity to dissolved humic substances. According to Visual MINTEQ 3.1, Cr is subjected to redox reactions, being Cr (VI) present (at low concentrations) in the studied rivers. According to Bio-met model, the bioavailability of Cu and Zn is highly influenced by soluble organic matter and water hardness, respectively. For most PTEs, the bioavailability estimated by deploying DGTs in river waters tends to be slightly lower than the estimation obtained with speciation models, since in real conditions more environmental factors take place comparing to the finite number of parameters considered in models.

Transcriptional responses of zebrafish to complex metal mixtures in laboratory studies overestimates the responses observed with environmental water

Metals released into the environment continue to be of concern for human health. However, risk assessment of metal exposure is often based on total metal levels and usually does not take bioavailability data, metal speciation or matrix effects into consideration. The continued development of biological endpoint analyses are therefore of high importance for improved eco-toxicological risk analyses. While there is an on-going debate concerning synergistic or additive effects of low-level mixed exposures there is little environmental data confirming the observations obtained from laboratory experiments. In the present study we utilized qRT-PCR analysis to identify key metal response genes to develop a method for biomonitoring and risk-assessment of metal pollution. The gene expression patterns were determined for juvenile zebrafish exposed to waters from sites down-stream of a closed mining operation. Genes representing different physiological processes including stress response, inflammation, apoptosis, drug metabolism, ion channels and receptors, and genotoxicity were analyzed. The gene expression patterns of zebrafish exposed to laboratory prepared metal mixes were compared to the patterns obtained with fish exposed to the environmental samples with the same metal composition and concentrations. Exposure to environmental samples resulted in fewer alterations in gene expression compared to laboratory mixes. A biotic ligand model (BLM) was used to approximate the bioavailability of the metals in the environmental setting. However, the BLM results were not in agreement with the experimental data, suggesting that the BLM may be overestimating the risk in the environment. The present study therefore supports the inclusion of site-specific biological analyses to complement the present chemical based assays used for environmental risk-assessment.

Evaluation of free/labile concentrations of trace metals in Athabasca oil sands region streams (Alberta, Canada) using diffusive gradient in thin films and a thermodynamic equilibrium model

The Athabasca's oil sands exploitation is controversial due to its potential risks to water quality but little is known about the temporal changes in the most bioavailable fraction of metal, the free/labile species. In this study, diffusive gradient in thin films (DGT) and the Windermere Humic Aqueous Model (WHAM VII) equilibrium model were used to examine the temporal changes in free/labile metal (Cu, Ni, Zn, Pb) species in three tributaries of the north-flowing Athabasca River in the Athabasca oil sands region (AOSR). The influence of dissolved organic matter (DOM) composition (i.e. fulvic: humic ratio) on modeled Cu and Ni speciation showed a negligible effect on the labile concentration. The best agreements (92 ± 8%) between DGT-labile and WHAM calculated labile concentrations were found assuming the formation of iron oxyhydroxides (FeO(OH)). The agreement was only 70 ± 7% in the presence of inorganic colloidal aluminum oxyhydroxides (AlO(OH)) and in the absence of any inorganic colloids. Together these results suggest that a change in DOM composition had limited impacts on modeled free metal ion concentrations. Although the concentration of the main metal ligand (i.e. DOM), varied from 9 to 40 ppm, no significant temporal differences in the abundance of WHAM-modeled labile species were found, suggesting mobility and bioavailability of Cu, Ni, Pb and Zn were comparable over the 2003-2012 period.

Evaluation of the measurement of Cu(II) bioavailability in complex aqueous media using a hollow-fiber supported liquid membrane device (HFSLM) and two microalgae species (Pseudokirchneriella subcapitata and Scenedesmus acutus)

The environmental bioavailability of copper was determined using a hollow-fiber supported liquid membrane (HFSLM) device as a chemical surrogate and two microalgae species (Scenedesmus acutus and Pseudokirchneriella subcapitata). Several experimental conditions were studied: pH, the presence of organic matter, inorganic anions, and concomitant cations. The results indicated a strong relationship between the response given by the HFSLM and the microalgae species with free copper concentrations measured by an ion selective electrode (ISE), in accordance with the free-ion activity model (FIAM). A significant positive correlation was evident when comparing the bioavailability results measured by the HFSLM and the S. acutus microalga species, showing that the synthetic device may emulate biological uptake and, consequently, be used as a chemical test for bioavailability measurements using this alga as a biological reference.

Source identification and health risk assessment of metals in indoor dust in the vicinity of phosphorus mining, Guizhou Province, China

An investigation was performed to identify the sources of arsenic (As) and heavy metals in house dust and to assess the associated human health risks in the vicinity of phosphorus (P) mining in Guizhou, China. The concentrations and spatial distributions of mercury (Hg), As, cadmium (Cd), lead (Pb), iron (Fe), copper (Cu), manganese (Mn), and P in 23 house dust samples from the study area were determined. Greater concentrations of As and Pb were found compared with values in other investigations in various countries. Pollution sources were identified using multivariate statistical analysis. As, Pb, Mn, and Hg pollution was mainly attributed to mining activities, and Mn and Cd levels were largely associated with automobile emissions. The dominant wind direction and the distance of the residence from the mining region were found to play an important role in element distributions. A health risk assessment showed that As and Pb should be paid more attention, although the noncancer risks of the studied elements were within the safe range and the cancer risks of As and Cd are within the acceptable range under present conditions.

Effects of sediment geochemical properties on heavy metal bioavailability

As the largest container and resource of metals, sediment has a special role in the fate of metals. Factors influencing bioavailability of heavy metals in sediment have never been comprehensively considered and the sediment properties still fail to understand and even controversial. In this review, the mechanisms of sediment properties such as acid-volatile sulfides (AVS), organic matter, texture (clay, silt or sand) and geology, organism behaviors as well as those influencing the bioavailability of metals were analyzed. Under anoxic condition, AVS mainly reduce the solubility and toxicity of metals, while organic matters, Fe-Mn oxides, clay or silt can stabilize heavy metals in elevated oxidative-reductive potential (ORP). Other factors including the variation of pH, redox potential, aging as well as nutrition and the behavior of benthic organism in sediment also largely alter metals mobility and distribution. These factors are often inter-related, and various toxicity assessment methods used to evaluate the bioavailability of trace metals have been also discussed. Additionally, we expect that some novel synthetic materials like polysulfides, nano-materials, provide the substantial amendments for metals pollution in sediment.